Soldering remains the method of choice for bonding an electrical component to the surface of a substrate, such as a printed circuit board. Different processes are employed to bond different types of components. For example, a leadless surface-mounted component (e.g., a chip carrier) having metal pads on its exterior for bonding to metallized areas on a circuit board, is attached to the board by first depositing a volume (i.e., a "bump") of solder on each pad. Following application of the solder bumps, the chip carrier is then adhered to the circuit board so each solder bump is in aligned contact with a corresponding metallized area on the surface of the board. Finally, the chip carrier and the circuit board are heated to reflow the solder bumps so that the bumps bond the pads on the chip carrier to the metallized areas on the board.
The "solder bump" bonding process described above may also be used to attach each bond site on a semiconductor chip to a corresponding bond site on the surface of a semiconductor wafer. First, solder bumps are applied to the bond sites on the wafer and on the chip. The chip is then placed with its solder bumps in aligned contact with the solder bumps on the wafer, and thereafter, the solder bumps are reflowed to form a solid mechanical and electrical bond between the chip and the wafer.
The overall reliability of the solder bonds created by the solder bump process is adversely affected by missing and defective bumps. For example, if a solder bump is missing, or is of an insufficient height, then no electrical connection is made between the corresponding pad on the chip carrier and the circuit board or between the bond site on the semiconductor chip and on the wafer. Bridging between two adjacent solder bumps creates a unwanted short circuit. Solder bumps which are spaced too close together may later bridge during reflow, also giving rise to an unwanted short circuit.
Other types of undesirable defects include poor wetting caused by a pad or bond site which is untinned prior to application of the solder bump. This type of defect generally reduces the strength of the resultant solder bond. Defects such as "cold" (i.e., grainy) solder bumps, and bumps which have surface deformations (e.g., holes, pits or cavities) may cause an otherwise good solder joint to fail prematurely. Solder spatter or debris, characterized by the adhesion of isolated globules of solder to the chip carrier, or to the semiconductor chip or wafer following application of the solder bumps, is undesirable since it may lead to short circuits.
In an effort to detect defects of the type described above, I developed a solder bump inspection system which is described in U.S. Pat. No. 4,688,939, issued on Aug. 25, 1987, to AT&T Technologies Inc., and which is also described in my article "Automated Visual Inspection of Solder Bumps," published in the AT&T Technical Journal, March/April 1988, Vol. 67, Issue 2, at pages 47-60. The solder bump inspection system described in my patent and my article (both incorporated by reference herein) comprises a television camera trained on an electronic component undergoing inspection such that the optical axis of the camera is substantially normal to the surface of the component. A ring light illuminates the article's surface with "dark field" illumination by directing light at an angle with the surface from all sides. When the light is directed at the surface in this manner, only the light striking the curved surface of a specular, three-dimensional feature, such as solder bump, is reflected into the television camera. A machine vision processor processes the output signal of the camera to provide a one-dimensional plot of the cumulative intensity of the light reflected from the surface as a function of the lateral distance therealong.
From such a plot of the reflectance intensity (known as a profile), my previous solder bump inspection system can reveal defects such as missing, bridged, and excessive bumps. Further, by re-positioning of the ring lamp to provide "bright field" illumination (i.e. light striking the surface of the article substantially normal to its plane), poor wetting can also be detected. As useful as my previous solder bump inspection system is, there are certain defects which it cannot readily inspect. For example, subtle defects such as cold ("grainy) solder, near shorts, solder spatter, squashed bumps and bumps whose surface topography is deformed, i.e., the bumps are concave, or their surfaces contain pits, holes or cavities, cannot be easily detected by my previous system.
Thus, there is a need for an improved technique for inspecting solder bumps in order to detect as many different types of defects as possible.